1. Field of the Invention
The present invention is directed to a method and a device for detecting free fall of a mobile device.
2. Description of the Related Art
Such methods and devices are generally known. Portable electronic devices having integrated hard drives are increasingly being offered in the entertainment industry. During use, such devices may be exposed to impacts or drops, which may result in damage to the storage disks and accordingly a loss of data during operation of the hard drive due to the associated movement of the write-read heads. To prevent such damage, it is customary to use triple-axis acceleration sensors which are able to detect impacts and thus offer the possibility of moving the write-read head into a park position in good time. Free fall is detected if an acceleration of near zero is measured on all three axes. If such a case of free fall is detected, it is possible to park the hard drive of the device before impact of the device, i.e., the read head of the hard drive is brought into an area in which it is no longer located above the magnetic disk. Damage to the hard drive caused by the read head may thus be prevented. Roughly three times the mean access time is needed for parking the drive, so that the parking time ranges approximately from 40 to 200 ms.
An acceleration sensor capable of measuring acceleration signals separately on three orthogonal axes (x, y and Z axis) is used for detecting that a device is in free fall. As long as a device is not in motion, the length of the acceleration vector formed from x, y and z is (v=sqrt(x^2+y^2+z^2))=1 g (gravitational acceleration). In free fall, this value is not met and is located in an ideal sensor system, for example, at 0 g in a linear fall movement. The detection of free fall on the basis of a threshold v not being met is described, for example, in U.S. Pat. No. 5,227,929. In this method, it is necessary to calculate sums of squares and form a square root from each of these sums of squares. These numerous calculations are associated with a considerable expense.
Only three individual sensor channels are observed in U.S. Pat. No. 7,191,089 and U.S. Pat. No. 7,450,332. The linking is via AND logic and a check is performed to determine whether all three channels fall short of a threshold. In an ideal sensor, the lowest acceleration value in a channel without free fall and dynamics is equal to 1/sqrt(3).
The disadvantage of the forenamed methods according to the related art is that these methods react sensitively to sensor parameter scattering and to less than ideal sensor characteristics. Furthermore, sensors according to the related art require a comparatively large amount of space and result in comparatively high costs. In addition, it is only possible to implement sensors according to the related art in a microcontroller, making it impossible to have a comparatively simple circuit directly in the sensor.